Some 15,000 patients will be able to buy 'top-up' drugs privately without losing their right to free NHS care under landmark changes to be announced today.

Under current rules, patients who buy life-extending drugs that are not approved for the NHS face a huge bill from the health service for the rest of their care.

This threat will be removed following a historic change of heart by the Government.

But charities warn that cancer patients who choose to pay privately will still end up with average bills of £21,000 for the drugs.

The U-turn follows a public outcry over terminally ill people being hit with huge NHS bills in their final years.

Health Secretary Alan Johnson will spell out the change to MPs today.

The drugs involved have been turned down or have yet to be approved by the rationing watchdog, the National Institute for health and Clinical Excellence.

They include Erbitux for bowel cancer, Rituximab for Non-Hodgkin lymphoma, Velcade for multiple myloma and Nexavar, Sutent and Avastin for kidney cancer.

Mr Johnson will also outline a review by cancer czar Professor Mike Richards, which is expected to include wholesale reforms to NICE designed to increase access and 'risk sharing' schemes with drugs firms.

The top-up drug situation is the result of guidance that each 'episode of care' for a patient in an NHS hospital must be paid for totally by the state or totally from private income.

This has been interpreted to mean that people who pay for their drugs cannot have free NHS care because they would then be a 'mixed' patient.

But charities representing cancer patients and others who want access to banned drugs say an estimated 8,000 to 15,000 a year ask local health trusts for treatment under an 'exceptional cases' procedure.

Last month the Rarer Cancers Forum said providing the drugs needed by 8,000 patients - the lower end of the estimate of people involved - would be only £175million, a tenth of the current NHS surplus.

Penny Wilson-Webb, chief executive of the forum, said: 'This review must ensure that the latest clinically-effective treatments are available to patients free of charge on the NHS.

To avoid top-ups, we need to get NICE working faster, we need to get exceptional cases panels working better, and we need to have fairer pricing of new drugs.

'The NHS should be there for all who need it, not to issue £20,000 bills to cancer patients for treatments recommended by their NHS doctor.'

The new move is hugely controversial among many Labour backbenchers who fear it is the thin end of the wedge to a 'two-tier' NHS - where people who can afford better treatment can pay for it.

The concerns are shared by the NHS Confederation, which represents health trusts.

LibDem spokesman Norman Lamb said: 'It is unacceptable to continue to deny people the right to top up their care, particularly where they are following their doctor's advice.

'There must also be reform of NICE to improve access to drugs which are routinely available across Europe but denied here.'